1. Field of the Invention
The present invention relates generally to a circuit for providing round robin arbitration, or round robin/modified priority arbitration, between requests for access to a shared resource which is normally provided with only priority arbitration between different access requests.
More particularly, the present invention is specifically described with the shared resource being a data bus, and provides a first option for a round robin arbitration system between different bus access requests, as opposed to a normal priority arbitration system, and a second option to put one bus request on a priority basis while other bus requests are round robin arbitrated. However, the present invention is also applicable to other shared resources such as a shared main storage, a shared RAM memory, etc.
Bus arbitration between masters for buses such as the Versa Module European (VME) bus can be accomplished by a commercially available MC68452 Bus Arbitration Module (BAM). The MC68452 Bus Arbitration Module is a bipolar asynchronous bus controller module which allows multiple local MPU buses to be multiplexed onto a common global bus, enabling the local buses to share memory, I/O devices, and communicate with each other easily and efficiently. It performs arbitration for up to eight users of a global bus, is expandable, and implements a fixed physical priority between bus requests. The commercially available Motorola 68452 Bus Arbitration Module only provides for priority arbitration between different bus access requests.